1. Field of the Invention
The present invention relates to a double-wavelength optical head device which is used for the recording/reproducing of optical recording media such as CDs (compact disks) and DVDs (digital versatile disks). More specifically, the present invention relates to, in such a double-wavelength optical head device, a beam-guiding optical device for changing the relative angle between the beams of different wavelengths emitted by different light sources having different emitting points, to bring the beams together onto the same optical axis or to guide the beams onto a common photo receiving device.
2. Related Art
CDs and DVDs are known as optical recording media and differ in substrate thickness and recording density. Such optical recording media require laser beams of different wavelengths to record and reproduce data thereon/therefrom. For example, DVDs need a laser beam having a wavelength of 650 nm for reproduction due to the low index of reflection within the 650 nm wavelength band whereas CD-Rs need a laser beam having a wavelength of 780 nm for recording and reproduction.
Further, an optical head device for reproducing DVDs and reading and/or writing CD-Rs is called a double-wavelength optical head device. A double-wavelength optical head device has both a laser light source for emitting a laser beam having a wavelength of 650 nm and another laser light source for emitting a laser beam having a wavelength of 780 nm. An example of such a double-wavelength optical head device is disclosed in Japanese Patent Application No. H8-55363.
In such a double-wavelength optical head device, the optical system is shared between the laser beams so that the device is compact. Therefore, the laser beams emitted from the different emitting points of the different laser light sources need to be guided to the same optical path through a beam splitter and converged onto an optical recording medium through a common objective lens. Also, the reflection from the optical recording medium is detected by a common photo receiving device.
However, the beam splitter used for guiding multiple laser beams to a common optical path reflects one of the laser beams and transmits the other laser beam to bring the beams together onto the same optical path. To converge the laser beams as a diffraction-limited spot through an objective lens for data reading, the wave aberration of the laser beam passing through the beam splitter should not be deteriorated. As such, a cubic prism is generally used as the beam splitter. The beam splitter is thus expensive and is an obstacle to reducing manufacturing costs.
Furthermore, when a common photo receiving device is used, the position of each of the two individually arranged laser light sources needs to be adjusted 3-dimensionally with respect to the common photo receiving device, and the beams emitted by the different laser light sources are reflected at the optical recording medium and then focused onto a spot. Thus, the cost of the positional adjustment is an additional problem. Also, reliability may be degraded.
In view of these problems, it is a purpose of the present invention to provide in an optical head device that has multiple light sources for emitting beams of different wavelengths, a beam-guiding optical device for bringing the beams emitted by the different light sources together onto the same optical axis and to do so while reducing manufacturing costs.
It is another purpose of the present invention to provide in an optical head device that has multiple light sources for emitting beams of different wavelengths, a beam-guiding optical device for guiding the beams reflected from the optical recording medium onto a common photo device which can be manufactured inexpensively.
Another purpose of the present invention is to provide an optical head device in which an optical system can be configured inexpensively and the adjustments thereof at installation can be easily performed by using a beam-guiding optical device that is inexpensively manufactured.